Window portion with an adjusted rate of wear

ABSTRACT

A polishing pad includes a polishing layer, and the transparent window portion of the polishing layer having dispersed particles to increase the rate at which the window portion wears away during a polishing operation, and to avoid forming a lump in the polishing layer.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of provisional applicationserial No. 60/189,386, filed Mar. 15, 2000.

FIELD OF THE INVENTION

[0002] The invention relates to a polishing pad having a transparentwindow portion in a polishing layer.

DISCUSSION OF RELATED ART

[0003] A polishing operation is performed on a semiconductor wafer toremove excess material, and to provide the wafer with a smooth, planarpolished surface. To attain the smooth, planar polished surface, thepolishing layer of the polishing pad provides a uniform polishingaction. During the polishing operation, polishing pressure is exerted onthe window portion and on the remainder of the polishing layer.

[0004] U.S. Pat. No. 5,893,796 discloses a known polishing pad having atransparent window portion installed in a polishing layer of thepolishing pad. It has been found that the window portion was fabricatedwith materials that have an inherent resistance to wear. Other materialsin a remainder of the polishing layer have a lower resistance to wear.Thus, as a polishing layer slowly wears away as it is being used topolish a semiconductor wafer, the transparent window portion wears awaymore slowly, at a lower rate of wear. As a result, the transparentwindow portion becomes a lump on the polishing layer, the lump having aheight greater than the height of the remainder of the polishing layer.

[0005] The window portion, being a lump on the polishing layer, ispressed inward by the polishing pressure to become flush with thepolishing surface. However, the inwardly pressed window portion polisheswith a different polishing action than that of the remainder of thepolishing layer. For example, the window portion, as a lump,concentrates polishing force against the semiconductor wafer, whichproduces a nonuniform polishing action. Consequently, the nonuniformpolishing action produces defects in the smooth, planar polished surfaceon the semiconductor wafer.

[0006] A need exists for a polishing pad having a polishing layer with atransparent window portion, which provides a uniform polishing action asthe polishing layer undergoes wear during a polishing operation.

[0007] Further a need exists for a transparent window portion thatavoids becoming a lump on a worn polishing layer of a polishing pad.

SUMMARY OF THE INVENTION

[0008] According to the invention, a transparent window portion of apolishing layer is provided with dispersed particles of at least one, ormore than one, substance dispersed throughout the window portion toincrease the rate at which the window portion wears away during apolishing operation and to avoid forming a lump in the polishing layer.

[0009] Embodiments of the invention will now be described by way ofexample with reference to the following detailed description.

DETAILED DESCRIPTION

[0010] Embodiments of the invention will now be described by way ofexample with reference to the following detailed description.

[0011] A semiconductor wafer having integrated circuits fabricatedthereon must be polished to provide a very smooth and flat wafer surfacewhich in some cases may vary from a given plane by as little as afraction of a micron. Such polishing is usually accomplished in achemical-mechanical polishing (CMP) operation that utilizes a chemicallyactive slurry that is buffed against the wafer surface by a polishingpad. Methods have been developed for determining when the wafer has beenpolished to a desired endpoint. According to U.S. Pat. No. 5,413,941,one such method includes light generated by a laser to measure a waferdimension.

[0012] According to a known polishing pad, the surface of thetransparent window portion is flush with the polishing surface of thepolishing pad. The window portion and the polishing surface are incontact with the workpiece, i.e. semiconductor wafer, being polished.

[0013] When the window portion has a wear rate that is lower (i.e., itwears slower) than that of the polishing surface surrounding it, thepolishing layer wears away at a rate that is faster than the rate atwhich the window portion wears away. The height of the window portionbecomes greater than the height of the polishing layer. The performanceof the polishing pad is jeopardized.

[0014] A polishing operation is performed on a semiconductor wafer toremove excess material, and to provide the wafer with a smooth, planarpolished surface. To attain the smooth, planar polished surface, thepolishing layer of the polishing pad provides a uniform polishingaction. During the polishing operation, polishing pressure is exerted onthe window portion and on the remainder of the polishing layer. Thewindow portion, being a lump on the polishing layer, is pressed inwardby the polishing pressure to become flush with the polishing surface.However, the inwardly pressed window portion polishes with a differentpolishing action than that of the remainder of the polishing layer. Forexample, the window portion, as a lump, concentrates polishing forceagainst the semiconductor wafer, which produces a non-uniform polishingaction.

[0015] Examples of such pads include urethane impregnated polyesterfelts, microporous urethane pads of the type sold as Politex® by Rodel,Inc. of Newark, Del., and filled and/or blown composite urethanes suchas IC-series and MH-series polishing pads also manufactured by Rodel,Inc. of Newark, Del. Window portions used in these types of urethanepads typically comprise urethane with the standard additives in thePolitex® and IC- and MH-series.

[0016] A known polymeric pad has a matrix that comprises materialsselected from polyurethanes, acrylics, polycarbonates, nylons,polyesters, polyvinyl chlorides, polyvinylidene fluorides, polyethersulfones, polystyrenes, and polyethylenes, polyurethanes, acrylics,polycarbonates, nylons, and polyesters with higher wear rates than thecurrently used polyurethanes.

[0017] A known polymeric matrix that can be used according to theinvention comprises materials selected from polyurethanes, acrylics,polycarbonates, nylons, polyesters, polyvinyl chlorides, polyvinylidenefluorides, polyether sulfones, polystyrenes, polyethylenes, FEP, TeflonAF®, and the like. Other materials are polyurethanes, acrylics,polycarbonates, nylons, polyesters and polyurethanes. Further examplesinclude polymethylmethacrylate sheets (e.g., Plexiglas® sold by Rohm andHaas, Philadelphia, Pa.) and polycarbonate plastic sheets (e.g., Lexan®sold by General Electric). Casting or extruding the polymer and thencuring the polymer to the desired size and thickness can make the windowportions.

[0018] The polishing pad comprises a polymeric matrix formed fromurethanes, melamines, polyesters, polysulfones, polyvinyl acetates,fluorinated hydrocarbons, and the like, and mixtures, copolymers andgrafts thereof. The polymeric matrix comprises a urethane polymer. Theurethane polymer is advantageously formed from a polyether-based liquidurethane, such as the Adiprene™ line of products that are commerciallyavailable from Uniroyal Chemical Co., Inc. of Middlebury, Conn. Forexample, a liquid urethane contains about 9 to about 9.3% by weight freeisocyanate. Other isocyanate bearing products and prepolymers may alsobe used. The liquid urethane is advantageously one which reacts with apolyfunctional amine, diamine, triamine or polyfunctional hydroxylcompound or mixed functionality compounds such as hydroxyl/aminesdwelling in urethane/urea crosslinked networks to permit the formationof urea links and a cured/crosslinked polymer network. The liquidurethane is reacted with 4,4′-methylene-bis(2-chloroaniline) (“MOCA”),which is commercially available as the product CURENE® 442, fromAnderson Development Co. of Adrian, Mich.

[0019] Forming a window portion comprising a phase separated or biphasicsystem is accomplished by blending two immiscible polymers until theirdomain size will not scatter light and then polymerizing them in theshape of a window portion. The immiscible polymer is expected to providea window portion with particulates of immiscible polymer providing anincreased WR. Pairs of immiscible polymers can include, but are notlimited to, polyurea/polyurethane, nitrocellulose/acrylic and the like.

[0020] If the wear rate (WR) of the transparent window portion is equalto or greater than the WR of the polishing surface, then the windowportion will be expected to remain flush with the polishing surfaceduring a polishing operation. Wear rate is a measure of how quickly thesurface of the window portion surface or polishing surface is removed,or worn away, during chemical-mechanical polishing. Abrasion resistance,or resistance to abrasion, is a measure of how the surface of the windowportion or of the polishing surface avoids being removed or worn away byabrasion during chemical-mechanical polishing. The invention provides atransparent window portion that has a higher wear rate and lowerabrasion resistance than window portions fabricated with materialshaving inherently high resistance to wear, as in previous polishingpads. Advantageously, the WR_(window portion) is equal to or at least 5,10, 15, 20, 25, 50, 100, or 200% greater than WR_(pol surface). Moreadvantageously, the WR_(window portion) is 5, 10, 15, 20, to 25% greaterthan WR_(pol surface).

[0021] The invention provides a transparent window portion comprised ofa polymeric matrix further comprising a discontinuity that increases thewear rate (or decreases the abrasion resistance) of the window portioncompared with the polymeric matrix without the discontinuity.

[0022] Discontinuity, as used herein, is intended to mean that thepolymeric matrix has been disrupted by the presence of a foreignmaterial. A desired discontinuity is one that increases the WR of thepolymeric matrix. The amount of the disruption or discontinuity dependson the desired WR of the polymeric matrix. Discontinuities can beobtained by the forming the polymeric matrix in the presence of solidparticles, fluids, gases, or an immiscible polymer system. The polymericmatrixes are prepared so that the discontinuities do not mechanicallyreinforce the matrix or are so large as to cause scattering of anincident optical beam that prohibits optical end-point detection.Additives can include solid particles (e.g., silica, titania, alumina,ceria, or plastic particles). Advantageously the additives are plasticparticles. Nanometer sized particles, are particles of one nanometer andless in size, that are of sufficiently low surface area to avoidscattering of incident light. Dispersal of the particles in the windowportion, rather than agglomeration of the particles, further avoidsscattering of incident light.

[0023] The particles (e.g., plastic particles) can range in diameterfrom 1 nm to 200 μm, advantageously from 1 to 50 μm, more advantageouslyfrom 10-20 μm. The actual shape of the plastic particles is not limited.It can include chips, squares, discs, pucks, donuts, spheres, cubes,irregular shapes, etc. Advantageously, from 1, 2, 3, 4, 5, 6, 7, 8, 9 to10% of the weight of the window portion is from the solid particles.

[0024] The plastic comprising the particles is chosen depending on thepolymeric matrix of the window portion. The plastic is chosen such thatits presence has little or no effect on the index of refraction of thewindow portion. Advantageously the plastic has about the same index ofrefraction as the polymeric matrix of the window portion. Advantageouslythe plastic is the same as the polymeric matrix of the window portion.Thus, the plastic can be selected from polyurethanes, acrylics,polycarbonates, nylons, polyesters, polyvinyl chlorides, polyvinylidenefluorides, polyether sulfones, polystyrenes, and polyethylenes.Advantageously, the plastic is selected from polyurethanes, acrylics,polycarbonates, nylons, and polyesters. More advantageously, the plasticis polyurethane.

[0025] Fluids in the form of a polymeric emulsion are expected to createa discontinuity. By forming the window portion in the presence of afluid, a polymeric matrix can be obtained that encapsulates the fluid inindividual, spaced cells, including bubbles. This is expected toincrease the WR of the window portion. Advantageously, from 1, 2, 3, 4,5, 6, 7, 8, 9 to 10% of the weight of the window portion is from thefluid. For example, such fluids or liquids include hydrocarbon oils suchas mineral oil.

[0026] Another discontinuity can be the presence of a gas in thepolymeric matrix. By forming the window portion in the presence of a gastype fluid, a polymeric matrix can be obtained that encapsulates thefluid in individual, spaced cells, including bubbles. Advantageously,from 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98 to 99% ofthe volume of the window portion is a gas (e.g., air, carbon dioxide, ornitrogen). For example, an aerogel is a silica aerogel. The silicaaerogels are prepared from silicon alkoxides, advantageously tetramethylorthosilicate or tetraethyl orthosilicate.

[0027] The transparent window portion of the present invention should betransparent to light having a wavelength within the range of 190 to 3500nanometers, depending on the application and optical device being usedto monitor the polishing process. The transparent window portion shouldalso be transparent to allow for optical end-point detection of thedevice being polished.

[0028] Embodiments of the invention having been disclosed, otherembodiments and modifications of the invention are intended to becovered by the spirit and scope of the appended claims.

What is claimed is:
 1. A polishing pad for polishing semiconductorwafers, comprising: a polishing layer having a transparent windowportion; the transparent window portion being constructed of a wearresisting material with an inherent resistance to wear when subjected toabrasion during a polishing operation; the rate at which the windowportion wears away during the polishing operation being increased toavoid forming a lump in the polishing layer, by having dispersedparticles of at least one, or more than one, substance dispersedthroughout the window portion; and the particles providingdiscontinuities in the wear resisting material, which are susceptible towear when subjected to abrasion during the polishing operation.
 2. Thepolishing pad as recited in claim 1 wherein, the dispersed particles arepieces of solid matter having a lower resistance to wear than that ofthe wear resisting material.
 3. The polishing pad as recited in claim 1wherein, the dispersed particles are pieces of solid matter that aresusceptible to being snagged and worn away when subjected to abrasionduring the polishing operation.
 4. The polishing pad as recited in claim1 wherein, the dispersed particles are cells of entrapped fluid withinthe wear resisting material; and the cells are susceptible to beingsnagged and worn away when subjected to abrasion during the polishingoperation.
 5. The polishing pad as recited in claim 4 wherein, theentrapped fluid is air.
 6. The polishing pad as recited in claim 4wherein, the entrapped fluid is deionized water.
 7. The polishing pad asrecited in claim 4 wherein, the cells are in the form of bubblescontaining the entrapped fluid.
 8. The polishing pad as recited in claim1 wherein, the wear rate of the window portion is adjusted to be greaterthan that of the remainder of the polishing layer, which reduces a forceexerted normal to the window portion as the window portion wears awayduring the polishing operation.
 9. The polishing pad as recited in claim1 wherein, the wear rate of the window portion is adjusted to besubstantially equal to that of the remainder of the polishing layer,which evenly distributes a force exerted normal to the polishing layerduring the polishing operation.
 10. A method of polishing asemiconductor wafer with a polishing pad having a polishing layer with atransparent window portion, comprising the steps of: providing dispersedparticles of at least one, or more than one, substance dispersedthroughout the window portion to increase the rate at which the windowportion wears away during a polishing operation and to avoid forming alump in the polishing layer, and polishing the semiconductor wafer withthe polishing layer having the transparent window portion, and theparticles providing discontinuities in the wear resisting material,which are susceptible to wear when subjected to abrasion during thepolishing operation, without the window portion forming a lump in thepolishing layer.
 11. The method as recited in claim 10 wherein, the stepof providing the dispersed particles, further includes the step of:providing the dispersed particles as pieces of solid matter having alower resistance to wear than that of the wear resisting material. 12.The method as recited in claim 10 wherein, the step of providing thedispersed particles, further includes the step of: providing thedispersed particles as pieces of solid matter that are susceptible tobeing snagged and worn away when subjected to abrasion during thepolishing operation.
 13. The method as recited in claim 10 wherein, thestep of providing the dispersed particles, further includes the step of:providing the dispersed particles as cells of entrapped fluid; and thenanometer sized cells are susceptible to being snagged and worn awaywhen subjected to abrasion during the polishing operation.
 14. Themethod as recited in claim 10 wherein, the step of providing thedispersed particles, further includes the step of: providing thedispersed particles as cells of entrapped air.
 15. The method as recitedin claim 10 wherein, the step of providing the dispersed particles,further includes the step of: providing the dispersed particles as cellsof deionized water.
 16. A method of making a window portion of apolishing pad, comprising the steps of: providing a transparent windowportion of a polishing layer with dispersed particles of at least one,or more than one, substance to increase the rate at which the windowportion wears away during a polishing operation and to avoid forming alump in the polishing layer.
 17. A polishing pad useful for polishingintegrated circuit wafers, comprising: a polishing surface and atransparent window portion disposed in an opening in the polishingsurface, wherein the window portion has a wear rate equal to or greaterthan that of the polishing surface.
 18. A pad according to claim 17 ,wherein the window portion is comprised of a polyurethane having a wearrate of from 5 to 25% greater than the wear rate of the polishingsurface.
 19. A pad according to claim 17 , wherein the window portion iscomprised of polymethylmethacrylate or polycarbonate.
 20. A padaccording to claim 17 , wherein the window portion comprises adiscontinuity selected from solid particles, fluids and gases.
 21. A padaccording to claim 17 , wherein the window portion comprises apolyurethane and a discontinuity selected from solid particles, fluidsand gases.
 22. A pad according to claim 21 , wherein the discontinuityis a plastic particle.
 23. A pad according to claim 22 , wherein thediameter of the particle is from 10-20 μm.
 24. A pad according to claim21 , wherein the window portion comprises from 1 to 10% by weight of theparticles.
 25. A pad according to claim 21 , wherein the window portioncomprises polyurethane and the plastic particle is polyurethane.
 26. Apad according to claim 21 , wherein the discontinuity is a fluid.
 27. Apad according to claim 26 , wherein the fluid comprises from 1 to 10% byweight of the window portion.
 28. A pad according to claim 26 , whereinthe fluid is a hydrocarbon oil.
 27. A pad according to claim 26 ,wherein the fluid is mineral oil.
 27. A pad according to claim 21 ,wherein the discontinuity is a gas.
 28. A pad according to claim 27 ,wherein the gas is carbon dioxide, nitrogen, or air and the gascomprises 85 to 99% of the volume of the window portion.
 29. A padaccording to claim 17 , wherein the transparent window portioncomprises: a silica aerogel.
 30. A pad according to claim 29 , whereinthe silica aerogel is prepared from tetramethyl orthosilicate ortetraethyl orthosilicate.
 31. A pad according to claim 21 , wherein thediscontinuity is an immiscible polymer system.
 32. A pad according toclaim 31 , wherein the immiscible polymer system ispolyurea/polyurethane or nitrocellulose/acrylic.